1. Field of the Invention
This invention relates to ethylene copolymer based hot-melt adhesives which retain good adhesion at elevated temperatures, and more specifically it relates to such adhesives formulated from a blend of certain specified ethylene copolymers with thermoplastic block-polyester elastomers, together with a tackifier. The adhesives are particularly useful for use with unprimed polyvinyl chloride (PVC) based substrates.
2. Description of Related Art
Use of adhesives applied as a hot melt is growing rapidly because hot melts allow fast production speeds. However, for many applications, limited temperature resistance of hot-melt adhesives is a major deficiency. This is the result of seemingly contrasting requirements. Hot-melt application requires high fluidity at application temperatures, yet adhesive strength at high temperatures requires a strength which any fluidity would preclude.
While hot-melt adhesives with a broad range of viscosities are known, viscosities required for hot-melt application are considerably less than those employed in common plastic forming operations such as molding and extrusion. As a result, formulation with low viscosity additives is commonly employed. While such additives have other functions as well, they `dilute` the polymer properties. Use of lower molecular weight polymers also reduces viscosity but results in adhesives with less `guts`. The combination of fluidity, strength and high-temperature utility is thus a major challenge for hot-melt adhesives.
Adhesives are, to a significant extent, substrate specific. When an adhesive composition does not adhere well to a specific substrate, adhesion can be improved to some extent by `priming` the substrate. This is often the case with rigid PVC. However, it is more desirable and less expensive not to have to prime a substrate. When the substrate is not primed, adhesiveness to a substrate requires a certain chemical `compatibility` with that substrate. For PVC, known hot-melt adhesives include ethylene copolymers at the low-cost, low-performance end and certain polyamides and certain types of polyesters at the high-cost, high-performance end. Polyamide hot-melts, useful for PVC, have good temperature resistance. However they are expensive and are moisture sensitive.
Ethylene/vinyl acetate copolymers were among the earliest hot melt adhesives. They may be used as PVC adhesives, and with moderate to high levels of vinyl acetate adhere to PVC without priming the PVC. Terpolymers of ethylene, esters of unsaturated carboxylic acids and carbon monoxide have also been disclosed as adhesives for unprimed PVC and are described in Japanese Laid Open Application No. J3-160085.
U.S. Pat. No. 3,780,140 (Hammer) discloses such terpolymers as being `compatible` with PVC, and in certain composition ranges useful as plasticizers for PVC which, it is noted, implies actual molecular miscibility. Ethylene/n-butyl acrylate/carbon monoxide copolymer is exemplified.
Japanese Laid Open Application No. J3-181557 describes the same polymer in combination with a tackifier. PCT Patent Application No. WO 91/18043 describes use of ethylene/acrylate/carbon monoxide copolymers in combination with a tackifier as adhesives useful for a wide variety of substrates including PVC. These copolymer compositions however have limited utility as adhesives at higher temperatures.
Various means are used to counter the decreasing utility of hot-melt adhesives at higher temperatures. Curing or crosslinking after melt application is one approach of recent interest. Another approach is to use block or graft copolymers, with both a `soft` and a `hard` or `strong` segment, where the hard block or graft segment maintains hardness and strength to higher temperatures, while the soft segment is responsible for maintaining useful adhesive qualities at ambient and lower temperatures. Common amongst those that have found adhesive utility are styrene-based block copolymers which find use for both pressure sensitive and conventional hot-melt adhesives. Polyamide grafted ethylene copolymers are disclosed in EP 0455412, where the polyamide graft acts as a means of improving the high temperature performance of ethylene based copolymer adhesives. In both cases, the harder and more temperature resistant segment tends to act as a separate phase. The two segments are generally not compatible, and are held together by covalent bonds. In the EP application it is suggested that best high temperature adhesive properties are achieved when the hard phase is the continuous phase. Grafting however has the disadvantage of increasing the viscosity of the polymer melt, thereby imposing a greater difficulty in achieving the necessary high fluidity for hot-melt application. In addition, graft polymers may be relatively difficult and expensive to produce.
Mere blending two polymers such as a harder and more temperature resistant polymer with a soft adhesive polymer however requires a certain `compatibility` between the polymers of the blend will have no strength. High molecular weight polymers are often quite incompatible. If on the other hand the polymers are actually miscible, useful compositions may result but the harder more temperature resistant polymer will no longer maintain its independent behavior. Blending of polymeric materials as an approach to improving temperature resistance of hot-melts adhesives is not common. Blending is however a stock in trade of the adhesives industry, particularly for hot-melt adhesives. Such blending is typically with plasticizers, tackifiers and waxes. These are usually low molecular weight, low viscosity materials, designed to change various characteristics including increasing tackiness and reducing cost but particularly lowering the viscosity of the melt. Certain polyesters are well known as adhesives. Thermoplastic block-polyester elastomers however are generally high viscosity materials useful for many applications but are not known for use as adhesives. Blends of such block-polyesters with PVC are disclosed in U.S. Pat. No. 3,718,715 (Crawford et al.). The block-polyester is disclosed as being capable of functioning as a plasticizer for PVC.
Blends of such block-polyesters with ethylene copolymers are also known. U.S. Pat. No. 4,275,180 (Clarke) discloses such blends where the ethylene copolymer may contain units derived from a large variety of monomers including an alkyl acrylate and carbon monoxide. The choice of ethylene copolymer component is based on similarity in solubility parameter. The polymers are described as being useful as heat shrinkable elastomers with good resistance to hydrocarbons. No polymer containing carbon monoxide is exemplified. The blends are crosslinked and there is no suggestion the blends would be useful adhesives.
U.S. Pat. No. 4,739,012 (Hagman) discloses blends of a block-polyester elastomer with a partially crosslinked blend of an ethylene copolymer and polyvinyl chloride. The blend must contain polyvinyl chloride. While it is described as finding utility in adhesive and sealant applications, the viscosities of the blends are quite unsuitable for hot-melt adhesive applications.
There remains a need for an inexpensive way to improve the heat-resistance ethylene copolymer adhesives while maintaining sufficient fluidity for hot-melt application, excellent cohesive strength and excellent adhesivity to PVC.